This invention relates to preformed silica shapes and, more particularly, it concerns lime-free preformed shapes which are useful in the casting of molten aluminum.
Preformed refractory pins and spouts made from fused silica are typically used for casting molten aluminum into ingots. In the casting process, molten aluminum flows from a trough into a spout and from the spout into an ingot casting mold. The rate of metal flow is controlled by raising or lowering a pin which is located in the middle of the spout along its vertical axis (FIGS. 1 and 2). Fused silica is the preferred composition of these pins and spouts because of its inherently high thermal shock resistance and its relatively good resistance to molten aluminum. High thermal shock resistance is important in this application since the pins are thermally cycled when they are lowered into and raised out of the molten aluminum.
Typically, the above-mentioned silica pins and spouts are made by slip casting. In this process, fused silica is milled with water until the slip has a free flowing consistency. The slip has a solids content of about 89% and is sized -50 mesh. After casting into plaster of paris molds, the shapes are dried and fired to about 2000.degree. F. in order to develop a ceramic bond. The disadvantages of this operation are twofold: first, the casting is restricted to plaster molds which tend to have limited life because of their low strength and, second, firing of the shape to a high temperature is required in order to develop strength.
In light of the foregoing, there is a need for improved preformed silica shapes which are adapted to the casting of molten aluminum.